No coffee," I explained. "I never drink coffee after three forty-eight p.m. It interferes with sleep. Caffeine has a half-life of three to four hours, so it's irresponsible serving coffee at seven p.m. unless people are planning to stay awake until after midnight. Which doesn't allow adequate sleep if they have a conventional job. -- Don
(Chapter 1 paragraph 44)

Importance: Don lives a very regimented life with everything timed out and on schedule. Even Don's food choices are the same based on the day of the week. It is difficult for Don to adapt and being offered coffee at the lecture seems ridiculous for him because of the impact it would have on the rest of his evening which is planned down to the minute, as are all his evenings.